Ask HN: What is the best way to find out about competition for an idea?

I know that it's a pretty common desire on HN and in the startup community to find out what competition exists for a particular idea, but I don't know of any fool-proof ways of going about it. You can use a combination of startup indexes like Angelist.com and Techcrunch's Crunchbase, search through HN of course, and browse the app stores, but it's very easy to not find out about competition for a long time even employing these tactics. Does there exist one all-encompassing resource or directory that makes it easy to vet an idea?